1. Technical Field
The present invention relates to communications systems using portable telephones and more particularly to a system whereby portable telephones can readily be used in conjunction with a cellular radio network. Such cellular radio networks are now in operation in the United Kingdom.
2. Prior Art
Cellular radio networks of various types have been described by which a radio telephone service can be provided to a large number of mobile subscribers using a relatively small number of frequencies, by dividing the service area into a number of cells and reusing the frequencies in non-adjacent cells. One such system is described in Bell Systems Technical Journal Volume 58 January 1979 No. 1, particularly in papers entitled "Advance mobile phone service: Introduction, Background and Objectives" by W. I. Young and "The Cellular Concept" by V. H. MacDonald.
In order to provide the necessary power for a resonable period of operation, it is usually necessary for a mobile radio telephone unit to include relatively large batteries which are both bulky and heavy. While such batteries can be readily mounted in a vehicle, it is not practicable with present mobile telephones to provide a truly portable unit.
The mobile radio telephone unit also has to perform complex procedures in order to register with its closet base station and communicate with the base station in order to receive, set up or hand-off a call. The necessary equipment for carrying out these functions is complex and expensive.
British Patent Specification No. 1461624 suggests an arrangement in which portable radio telephone units can operate at very low power and therefore use smaller and lighter batteries. In this system a number of receiver stations are located throughout each cell and relay the low strength received signals from the portable telephones to the main transmitting cell site. Apart from its low power the proposed portable radio telephone has to perform all the other functions of a normal vehicle mounted type of cellular radio telephone unit.